Production pipe joints in today's very deep oil or gas producing wells must withstand pressures at the ground level of the order of 10,000 psi and at the bottom of the wells, considerably higher. Current API standards for joints of this type are drawn on the assumption that the materials of the pipe and the coupling are infinitely rigid. In other words, they assume that there will be no elastic elongation of the pipe cross section behind each pitch of the pipe threads. In reality, however, the pipe and to a lesser degree the coupling will both be elastically and often plastically deformed under the loading imposed by modern very deep wells. Both the present API and other past designs cause the load to be borne mainly by a small number of threads at the entrance of the coupling which imposes a very high stress concentration on these few threads and causes many pipe failures. Fluid pressure sealing of standard API production pipe joints depends on a high viscosity pipe thread sealing compound which was adequate several decades ago when the API standards were adopted. Such efforts at seals have been proven to be inadequate for the pressures encountered in the very deep oil and gas wells of today.
More recently, there have been some attempts to modify these pipe joints and one such attempt involves a double tapered female thread of the coupling member so that the entrance threads of the coupling have slightly smaller thread taper than the corresponding thread taper of the male thread. In practice, however, the wedging radial pressure between the female threads of the coupling with a reduced taper and the standard API male thread of the pipe end when torqued together will deform the coupling by plastic deformation which renders the coupling unfit for reuse. At the same time, this expedient does not provide sufficient axial load bearing capacity or torquing capability and results in poor radial sealing force. By way of example, for 27/8 inch size pipe, the API specifies a minimum tightening torque of 1800 FT/LB. With the design just described, however, plastic deformation (radial yield) of the pipe coupling limits the tightening torque to 1200 FT/LB.
In another recent coupling design the taper of the last 5 to 10 female threads is increased in an effort to obtain a metal to metal seal between the male and female threads. The difficulty with this approach has been that the end of the male pipe threads tend to yield in compression beyond the elastic limit thus rendering the joint incapable of reuse. Another difficulty with this type of joint is the increased danger of galling the threads and still not necessarily achieving a more effective fluid pressure seal.
Still another attempt to make the API tapered threaded joint pressure tight is by placing a glass fiber-filled TEFLON ring in an internal radial groove of the coupling. The radial parallel sides of the seal ring groove are supposed to confine the seal ring prior to and during the assembly operation. The difficulty with this design is that it has proved to be almost impossible to keep the TEFLON ring in place during the assembly operation. What usually happens is that the ring twists out of the parallel sided groove, the groove walls being perpendicular to the coupling axis. The assembly is necessarily a "blind" one and the faulty sealing is usually discovered only after the whole pipe string has been assembled and put into service.
To summarize the shortcomings of the prior art attempts to solve the problem, the previously known pipe coupling joints are based upon a localized, small area radial interference between the male and female threads. A localized interference of the radial threads deforms both female and male members to a point that the components cannot be reused for a second assembly. In addition, the pipe and/or the coupling are often coated from the inside with an anticorrosion laquer which tends to flake off whenever there is plastic yielding of the pipe and coupling. From a load bearing standpoint, the most highly stressed portion of the pipe is the interface of the pipe thread and the first few threads of the coupling which is also the most usual area for the origin of fatigue failure.